1. Field of the Invention
The present invention relates to a light beam scanning apparatus which is mounted to an image forming apparatus such as a copy machine or a laser beam printer, and an image forming apparatus provided with the same.
2. Description of the Related Art
Conventionally, in an electrographic image forming apparatus, generally, a semiconductor laser is driven to emit a laser beam, and the laser beam is deflected at a rotating multi-faceted mirror (polygon mirror) rotated by a scanner motor so as to scan and irradiates the scanned beam onto a photosensitive member based on input image data. In this way, a latent image is formed, which is developed to a toner image to be transferred onto a storage medium to form an image.
Recently, such image forming apparatuses have been developed to correspond to the advancement in the fields of printing and desktop publishing, and the image quality, especially the reproduction of high definition letters, line drawing images, and halftone, required in the fields cannot be achieved without the formation of a high contrast exposure image on a photosensitive member. The recent higher speed of rotation of polygon mirror, higher speed of image processing, and simultaneous scanning of a plurality of scan lines have accomplished higher speed and higher resolution of image formation, which is enhancing the resolution of an image. However, the contrast of exposure image which contributes to definition and quality of an image has not been significantly improved since a decrease of a beam diameter is limited due to the restriction of optical system.
In other words, the contract of exposure image can be improved by decreasing a laser beam diameter.
Generally, the decreasing of a laser beam diameter can be achieved, for example, by (1) shortening of laser wavelength, or (2) increasing of incident beam diameter D to an fθ lens, that is, increasing of a diameter of laser beam entering a polygon mirror.
In light beam scanning apparatuses, the above method (1), that is, the shortening of laser wavelength has been achieved by using infrared laser beam, red laser beam, and potentially, blue laser beam. However, under the present conditions, the wavelength of 660 nm of a red laser beam is not sufficiently short relative to the wavelength of 780 nm of an infrared laser beam, and even a blue laser beam has a wavelength of 400 nm which is about one half of that of an infrared laser beam. Meanwhile, the shortening of laser wavelength which is about one half of that of a typical laser beam brings another technical problem such that the focal depth on a photosensitive member surface is also decreased to one half of a typical depth, or the photosensitive member has a lower sensitivity to a shorter wavelength region.
When the above method (2) is applied to a light beam scanning apparatus, a rotating multi-faceted mirror should to have a larger diameter, which brings another problems such as an increased temperature of a motor and undesired noise due to increased windage loss.
As described above, a laser beam diameter could not be decreased in conventional light beam scanning apparatuses.
Then, a method for forming a latent image using multiple exposures has been proposed to solve the above problems.
For example, Japanese Laid-Open Patent Publication (Kokai) No H09-169136 discloses a method for forming an electrostatic latent image including a step of outputting an image signal generated by processing an image signal and inverted image signal generated by inverting the image signal, a step of emitting a first light beam having a predetermined intensity modulated according to the image signal and a second light beam having a intensity lower than that of the predetermined intensity modulated according to the inverted image signal, a step of combining the first light beam and the second light beam to form a combined light beam, and a step of scanning the combined light beam on a photosensitive member to form an electrostatic latent image.
Also, Japanese Laid-Open Patent Publication (Kokai) No. 2002-116395 discloses another method including a step of deflecting a plurality of light fluxes by using deflecting means, and a step of scanning the plurality of light fluxes in series on generally the same region on a scanned surface to provide the scanned surface with multi-valued light quantity.
In the above method using a combined light beam, a higher contrast is obtained by forming an electrostatic latent image on a photosensitive member by multiple exposure so that the image is provided with a sharp edge therearound, but the method has some disadvantages that the position accuracy of the combination of a first light beam and a second light beam cannot be permanently maintained in terms of thermal environment, and that the requirement of two light beams for one scan increases the complexity of configuration of the light beam scanning apparatus in order to correspond to the higher speed of image formation.
The above method using a plurality of light fluxes provides an advantage similar to that when the spot diameter of a light flux is decreased because the dot image which is developed from an electrostatic latent image formed by multiple exposure has a smaller diameter. However, in the method, one position in a main scanning direction is irradiated by different light fluxes, and it is hard to accurately irradiate the one position by the plurality of light fluxes due to optical face tangle errors of a polygon mirror, jitter of the motor for driving the polygon mirror, and misalignment between positions where the light fluxes transmit a lens.
In addition, in conventional light beam scanning apparatuses, the contrast of one dot is often reduced due to the scanning in a main scanning direction.
FIG. 8 is a view showing the construction of a general light beam scanning apparatus in the prior art.
In FIG. 8, an image forming apparatus 600 is configured with: a laser beam source 601 for emitting a laser beam modulated according to an image signal; a collimator lens 602 for converting the diffused light from the laser beam source 601 into a parallel beam; a cylindrical lens 603 for converging the laser beam into a sub scanning direction after the laser beam passes a collimator lens 602; a polygon mirror 605 which is driven to rotate by a motor (not shown); an Fθ lens 606 for collecting the laser beam which is deflected by the polygon mirror 605 and scanning the laser beam on a predetermined main scanning position at a constant speed; and a photosensitive drum 607 onto which an electrostatic latent image is formed by the scanning of the laser beam; and a laser driving circuit 608.
In the image forming apparatus 600, in an image formation at 600 DPI, when an image signal controller (not shown) receives an image signal which corresponds to one dot, the laser driving circuit 608 causes a laser beam to be emitted for about 10nS at a certain rotation number. While the laser beam is emitted, the laser beam scans a surface of a photosensitive drum in a main scanning direction, generally at about 40 μm (for 1 dot). As with the timing of emission, the emission of a laser beam starts 5 nS before the position where a dot is to be formed comes, and finishes 5 nS after the position where the dot is to be formed passes. FIGS. 9A to 9C are views showing an electrostatic latent image formed on a photosensitive drum in the manner described above. FIG. 9A shows spot profiles of an electrostatic latent image formed on a photosensitive drum as seen in the direction perpendicular to the surface of the photosensitive drum; FIG. 9B shows the distribution of light quantity in the spot profile of one dot in FIG. 9A; and FIG. 9C shows the distribution of light quantity in the spot profile of two dots in FIG. 9A. In FIGS. 9A to 9C, the reference numerals 701a and 701b illustrate a case where one dot is formed, while the reference numerals 702a and 702b illustrate a case where two dots are formed with one space being interposed therebetween.
However, when the laser beam produces a dot size of 60μm for example, the laser beam constantly moves at about 40 μm in a main scanning direction during the formation of one dot, resulting in that an electrostatic latent image having a width of 100μm is formed on a photosensitive drum, the image has the maximum light quantity point at its center. When two dots are formed with one space being interposed therebetween, two electrostatic latent images are superimposed at the space part, resulting in a light quantity distribution as shown as 702b of FIG. 9C. This causes a problem of reduced contrast of an electrostatic latent image formed on a photosensitive drum.